144 AGRICULTURE. 



X. MANURES AND FERTILIZERS. 



It is a matter of common experience among farmers that the 

 soil is impov'erished by continuous cropping, and the yields 

 obtained therefore gradually decreased. The decrease in 

 yields can only be prevented by applications of farmyard 

 manure or commercial fertilizers; ploughing and thorough 

 cultivation of the soil bring the land in a better mechanical 

 condition and increase the amount of available plant food 

 present in the soil, but these operations are not sufficient 

 to maintain the fertility of the land so that it will yield 

 equally well from year to year under otherwise favorable 

 conditions. Every crop harvested contains certain quan- 

 tities of fertilizing ingredients, and taking away these 

 amounts in general leaves the soil in a poorer condition for 

 the production of crops than it was before. 



The fertilizing ingredients of which the soil is thus liable 

 to be robbed are potash, phosphoric acid, nitrogen, and 

 sometimes lime. They are not present as such in the soil, 

 or in the fertilizers applied to the soil, but in chemical com- 

 binations with a large variety of compounds. The soil will 

 contain nearly all the different elements which chemists 

 have so far succeeded in isolating, but it is mainly the 

 three elements, potassium, phosphorus, and nitrogen, 

 which are apt to be decreased in the soil below the amounts 

 required for the nutrition of crops, or at least of maxi- 

 mum crops. In rational fertilization the effort therefore 

 always is to return to the soil such quantities of fertilizing 

 ingredients, in the shape of farmyard manure or com- 

 mercial fertilizers, as will restore the loss sustained by the 

 withdrawal of the crops harvested. Other mineral ingre- 

 dients contained in the crops need not generally be re- 

 turned to the soil, since they are nearly everywhere pres- 

 ent in abundance. 



